1. Field of the Invention
The present invention relates to a technique for obtaining original signal data by demodulating a signal which is modulated by signal data, and in particular to a technique suitably applicable to an apparatus.
2. Description of the Related Art
As an example, a technique for obtaining original signal data by comparing the magnitude of an original modulation signal with that of the threshold value generated from the average value of a modulation signal is known as a technique for demodulating a modulation signal which is ASK (Amplitude Shift Keying)-modulated by signal data.
FIG. 1 is a block diagram showing the configuration of a conventional demodulation circuit for demodulating an ASK modulation signal.
In the reception unit 100 shown in FIG. 1, a demodulation circuit 110 applies an ASK demodulation to the electric current value (an electric current is noted as “current” hereinafter) of the line L detected by a current detection unit 101. The demodulated signal is inverse-diffused to be spectrum diffusion-demodulated by a spectrum diffusion modulation unit 105 on the basis of a pseudo noise (PN) code generated by a PN code generation unit 104, and an inverse diffusion demodulation signal is output. In the meantime, a differential value detection unit 106 calculates the differential value of the aforementioned current value of the line L relative to the average value of the current value of the line L, and a weighting calculation unit 107 adds a weighting to the inverse diffusion demodulation signal on the basis of the differential value. Following the weighting, an integration unit 108 further integrates the inverse diffusion demodulation signal for each one symbol minute of transmission data so that a data judgment unit 109 performs a binary-judgment of the integration value to obtain the reception data.
The demodulation circuit 110 comprises an average value detection unit 102 and a comparison unit 103.
The average value detection unit 102 calculates and outputs the average value of the current value flowing in the line L. The comparison unit 103 compares the magnitude of the aforementioned average value output from the average value detection unit 102 with that of the current value flowing in the line L and outputs the comparison result as an ASK demodulation signal. Specifically, the comparison unit 103 uses the average value calculated by the average value detection unit 102 as reference value and outputs “1” if the current value flowing in the line L is no smaller than the aforementioned reference value and outputs “0” if it is smaller than the reference value.
Next is a description of FIG. 2 which exemplifies the specific circuit configuration of the demodulation circuit 110.
Referring to FIG. 2, all of transistors M101, M102 and M103 are p-MOS transistors and both of transistors M104 and M105 are n-MOS transistors.
A branch unit 111 generates a signal corresponding to a detection signal IDET which is ASK-modulated with signal data. The branch unit 111 comprises the transistors M101, M102 and M104. Here, the transistor M102 forms a current mirror in combination with the transistor M101 and sends a drain current equal to the current value of the detection signal IDET to the transistor M104. Therefore, a voltage, which is a constant value (noted as “constant” hereinafter) times the current value of the detection signal IDET, is generated at the gate of the gate transistor M104 whose drain and gate are in short-circuit.
The average value detection unit 102, comprising a resistor R101 and a capacitor C101, averages the gate voltage of the transistor M104 and applies the averaged voltage, that is, the average of the voltage value which is a result of multiplying the current value of the detection signal IDET by a constant, to the comparison unit 103.
The comparison unit 103 comprises the transistors M103 and M105 and inverters 112 and 113. Here, the inverters 112 and 113 are serially connected together, thus constituting a buffer.
The transistor M103, forming a current mirror in combination with the transistor M101 of the branch unit 111, outputs a current ISCOMP equal to the current value of the detection signal IDET. Meanwhile, the output voltage for the average value detection unit 102 is applied to the gate of the transistor M105 and therefore the transistor M105 applies the average value of the gate voltage of the transistor M104, that is, sends the average of the current value of the detection signal IDET as threshold current IthCOMP. Therefore, the output of the comparison unit 103, that is, the output of the inverters 112 and 113, which constitute the buffer, becomes “1” (i.e., a “high” level) if the current ISCOMP is no smaller than the threshold current IthCOMP, and becomes “0” (i.e., a “low” level) if the current ISCOMP is smaller than the threshold current IthCOMP.
Since the demodulation circuit 110 shown in FIG. 2, operating as noted above, outputs “1” when the detection signal IDET is no smaller than the average value of the present detection signal IDET, and outputs “0” when the detection signal IDET is smaller than the present average value, thereby possessing the function of demodulating an ASK modulation signal.
The relationship between the current ISCOMP and threshold current IthCOMP in the demodulation circuit 110 shown in FIG. 2 possibly varies if the characteristics among the transistors M101, M102 and M103 are not uniform and/or the characteristics between the transistors M104 and M105 are not uniform. The range of the variation of the threshold current IthCOMP relative to the current ISCOMP increases in proportion to the magnitude of the current value of the detection signal IDET input to the demodulation circuit 110. Because of this, when the current value of the detection signal IDET is large, even a change in the current ISCOMP does not possibly cause a change in the relationship in magnitude related to a varied threshold current IthCOMP if the ratio of the current amplitude (that is, the current amplitude of the detection signal IDET) of the current ISCOMP to the peak value of the current ISCOMP (that is, the current value of the detection signal IDET), this ratio is so-called a “modulation ratio”, is small as shown in FIG. 3. In such a case, an ASK modulation signal cannot be demodulated.
Here, let it think the case of, for example, inputting a detection signal IDET to the demodulation circuit 110 after the signal being simply amplified in order to increase a current amplitude relative to the current ISCOMP. FIGS. 4A and 4B respectively show the relationship between the current ISCOMP and threshold current IthCOMP prior to the amplification and the relationship between the current ISCOMP and threshold current IthCOMP after the amplification. As is clear from these figures, the inputting of the detection signal IDET to the demodulation circuit 110 after the signal is amplified certainly increases the current amplitude of the current ISCOMP. In this case, however, the range of variation relative to the threshold current IthCOMP also increases in association of increasing the current amplitude of the current ISCOMP, still disabling the demodulation of the ASK modulation signal.